In the field of recent digital high-speed radio communication systems, CDMA (Code Division Multiple Access) modulation and/or OFDM (Orthogonal Frequency Division Multiplexing) modulation are used. These multiplexed modulated waves based on the modulation schemes are characterized in that instantaneous power (peak factor, crest factor) is extremely high compared to the average power.
Therefore, the power amplifier for transmission that is provided in the transmitter of a radio communication apparatus is required to suppress leakage power to adjacent channels to a low level when amplifying a signal. Accordingly, the requirement is that the linearity of the power amplifier for transmission be kept up to an extremely high output level so as to suppress spreading of the transmission spectrum due to non-linear distortion.
However, power amplifiers having good linearity up to a level where the amplitude component is markedly large, tend to be large in size and become high in price and in power consumption.
For this reason, in general power amplifiers which exhibit good linear characteristics within a certain range in which the amplitude component is small but which present non-linear characteristics in the range in which the amplitude component is not smaller than the former range, are often used. When a power amplifier exhibiting non-linearity is used, if a large back-off is taken, the amplifier can keep up linearity in the operation range but these results in inefficiency. In contrast, if the back-off is made small, the amplifier can present high efficiency but will produce output signals having distorted waveforms. As a result, the distorted component of the radio transmission signal is prone to leak to adjacent channels.
To deal with this, various proposals in distortion compensation technologies have as their object the suppression of adjacent channel leakage that is caused by non-linear distortion in power amplifiers. Of these, the digital pre-distortion scheme can be mentioned as the distortion compensation technique that has been adopted most frequently in recent years.
Distortion compensation based on the digital pre-distortion scheme compensates for non-linear distortion in AM/AM characteristics (amplitude characteristics) and AM/PM characteristics (phase characteristics) arising in a power amplifier. The distortion compensation circuit digitally compares the transmission signal with the feedback signal to thereby obtain non-linear characteristics of the power amplifier. Then, the circuit determines the distortion compensating coefficient representing inverse characteristics of the non-linear characteristics and complex multiplies the transmission signal by that distortion compensating coefficient. The signal obtained by this complex multiplication is input to the power amplifier. As a result, the signal output from the power amplifier exhibits approximate linear characteristics.
JP2004-32609A and JP2008-258714 disclose circuits for compensating non-linear distortion by digital pre-distortion schemes.
The non-linear distortion compensation circuit disclosed in JP2004-32609A compensates for non-linear distortion by multiplying the input signal by a compensation value in conformity with the input amplitude. In particular, in this non-linear distortion compensation circuit, the memory capacity is reduced by previously storing distortion compensation coefficients in an alternate manner and interpolating the distortion compensation coefficients located between the stored distortion compensation coefficients.
The power amplifier disclosed in JP2008-258714A performs compensation by using compensation data output from a lookup table in accordance with the amplitude of the input data. In particular, in this power amplifier, the dynamic range of the amplifier is enlarged by using different lookup tables for a large signal region and for a small signal region, in accordance with the amplitude of the input data.